1. Field of the Invention
The present invention relates to an optical scanning device for writing latent images onto image holding members and a method for reducing noises in an optical scanning device.
2. Description of the Related Art
In a tandem color image forming apparatus, latent images are generally written onto four image holding members (e.g. photosensitive drums) disposed next to one another by irradiating them with light beams emitted from a plurality of light sources. The latent images are then developed into visible images with developers for different colors (e.g. toners in yellow, cyan, magenta, and black). Subsequently, a recording medium such as a recording paper held by a transferring belt is sequentially conveyed to a transferring unit of each of the image holding members so that the visible images in the different colors are superposed onto the transferring member. Finally, the images are fixed on the transferring member so that a multi-colored image is obtained.
In such a tandem color image forming apparatus, there is a conventionally-known technique to write latent images onto each image holding member individually by using optical scanning devices provided for each of the image holding members. However, such optical scanning devices that include a deflector (hereinafter also referred to as an optical deflector) that includes a polygon mirror and a motor for driving the polygon mirror are relatively expensive. Therefore, it is expensive to individually provide optical scanning devices for each of the image holding members.
In addition, it requires a large space to install as many optical scanning devices each including the same number of optical deflectors as that of the image holding members, which leads to a larger image forming apparatus.
In order to reduce the costs and the size of the tandem color image forming apparatus, a conventional optical scanning device includes a single optical deflector used commonly by a plurality of light sources, so that light beams emitted from the light sources are simultaneously deflected and scanned with the single optical deflector.
When scanning and image forming optical systems are disposed symmetrically on both sides of an optical deflector so that a single optical deflector can separate a plurality of light beams into two symmetrical directions and deflect and scan the light beams, a problem of flare beams emitted from the optical systems arises.
For example, when a light beam is reflected and scattered on the incident surfaces of image forming lenses that are opposed to each other on either side of an optical deflector positioned there between, the light beam reflected and scattered (called a flare beam) travels in the opposite direction so as to enter the optical system on the opposite side. Consequently, the flare beam that has entered the opposite optical system is irradiated onto one or more of the image holding members via the optical system.
If a flare beam is reflected onto an image holding member, it may cause stripe smudges or a ghost image in the resultant image, or sometimes it may cause smudges in the background or blurring of colors because of duplication of flare beams. Thus, flare beams significantly degrade the image quality.
Japanese Patent Application Laid Open No. 2002-196269 discloses an arrangement in which a light shielding member that is a flat plate and is used for shielding flare beams is disposed at a position close to a deflection surface of an optical deflector.
Japanese Patent Application Laid Open No. 2002-196269 also discloses a technique by which the light shielding member is placed so as to be out of alignment from a position on a line that orthogonally intersects the rotation center of the optical deflector and the positional directions of the optical systems, toward the downstream side of the rotation direction of the optical deflector, in order to reduce occurrence of noises including whistling noise that is caused when there is not enough distance between the optical deflector that rotates at a high speed and the light shielding member. Similar techniques are disclosed also in the Japanese Patent Application Laid Open No. 2003-185954, No. 2002-162592, No. 2003-202512, and No. 2003-255254.
In an optical scanning device that uses the oppositional scanning method as described above, because of the characteristics related to how the scanning and image forming optical systems are positioned, it is required to dispose a flare beam shielding member extremely closely to the optical deflector in order to efficiently shield flare beams.
However, when the flare beam shielding member is disposed extremely close to the optical deflector that performs deflection at a high-speed, noises increase, abnormal sounds are caused, and the temperature also rises. In some cases, deflection stability may be lowered.
Conventional techniques have not been able to achieve satisfactory results in terms of reducing side effects including noises.
In the above example, a flare beam shielding member is disposed close to an optical deflector; however, there might be a need to provide another unit other than the flare beam shielding member close to the optical deflector due to the layout of the optical scanning device. In such a case, a similar problem will arise.